1. Field of the Invention
The present invention concerns a method for the fast synchronization of vocoders coupled to one another by enciphering and deciphering devices.
2. Description of the Prior Art
In enciphering devices using vocoders, speech transmission between vocoders takes place digitally. The speech signals are first sampled and then encoded by the vocoder before being applied to the associated enciphering device. At reception, the samples of the speech signal enciphered by the enciphering device of the transmitter are deciphered and then applied to the reception vocoder.
To make the set thus formed work efficiently, it is necessary to see to the synchronization of the enciphering devices and as well as that of the transmission and reception vocoders. In known systems, a first cipher function synchronizing stage accurately initializes of the reception enciphering devices and a second stage synchronizes the vocoder function. This latter stage is undertaken only when the previous one is completed.
Since these two stages are performed sequentially, without the second stage being dependent in any way whatsoever on the first stage, the synchronizing of the vocoder/enciphering device set takes time.
In radio transmission mode, on HF channels for example, in the 1 to 30 Mhz frequency range, the average time taken to achieve synchronization is in the range of a few seconds. Since vocoders are generally used alternately between two speakers, or in conference mode among several speakers, the synchronization periods obtained are prohibitively long and are really troublesome for users.